Just a couple of things to add. If you are in the second row, please, please do not reach forward to grab the barrier. It is really painful being on the barrier with somones bony elbow digging into your ribs! On the same note, try to keep your elbows tucked in as much as possible because bony parts of bodies digging into backs is also very painful!
And, I think someone already mentioned, please be careful when putting your fist in the air not to hit the person in front in the head, the same when taking photos, make sure your arm is not in the person in front's face.
Always try to get to know the people around you too so you can look out for each other and if you see someone else having a bad time of it, look out for them.
Oh and above all else, HAVE FUN!!!